The present invention relates to circuits for starting and controlling the speed of a motor, and more particularly to a motor stating and speed controlling system including a technique for providing a ramp step for the system ramp function.
It is known that motor starting applications require that an induction motor or the like be started over a pre-selected period of time. It is desired that a limit be placed on the maximum current to be drawn by the motor during starting. In the motor control system referred to as an open loop voltage/frequency (V/F) control system, the frequency function and the voltage function control the motor speed, and more particularly the motor starting time and current that is drawn. For a given control device, the current is limited by the hardware.
Consequently, if the current of the motor can be maximized under the limit, a better starting time can be obtained. The ramp step of the current curve determines the motor current. If an optimum ramp step can be achieved, the maximized motor current can be obtained.
An object of the present invention is to provide an improved motor control system for a three-phase induction motor.
Another object of the present invention is to provide a motor control system including a motor starter and speed controller circuit wherein an induction motor speed is changed in accordance to a speed demand.
A further object of the present invention is to provide a motor control system wherein a ramp function is employed in a speed controller circuit to increase or decrease the frequency of an input signal to the motor in accordance with a speed demand.
Still another object of the present invention is to provide a motor control system wherein a PI (phase/current) controller is used in a speed controller circuit to control a ramp step applied to a ramp function to provide optimum performance of the motor control system.